Case Studies of Residential Foundation Movements in Southern Houston Area
After completing his doctoral work in August, 2004, Dr. Xiong Zhang put together a three-dimensional transient simulation of the deformations and stresses and strains in an L-Shaped residence with walls, windows, and doors under the influence of a tree and its root zone responding to the weather patterns in Arlington, Texas. This work was the first of its kind in modeling movement of a residential foundation.
Dr. Zhang developed five models to run the analysis which Dr. Lytton presented to an audience of 70.
hydro-mechanical stress model
The simulation was run on Texas A&M's super computer and covered two years of the entire building responding to the movements of the foundation soil as a result of the climate. The software used as ABAQUS, a 3D Finite Element Analysis program that allows water to move through the soil elements and element stiffness that can vary throughout the analysis. Contact (jointed) elements were used to simulate the soil-structure interaction. The foundation and superstructure were modelled using general shell elements.
The software was set up to solve rather complex partial differential equations at each joint for each time step. The time step used was a 24-hour day so that 730 time steps were analyzed, using approximately 24 hours of super computer run time for each problem. Two problems were analyzed, with the difference being the tree location relative to the foundation.
The slab was taken from one of the example problems (Appendix A.4) in the soon-to-be-published Post-Tensioning Institute 3rd Edition Design Manual. The 4" slab with 12"x34" deep grade beams was simplified as a 16" uniform slab that gave similar stiffness. The laboratory tests for the Arlington TX site that were input into the analysis included liguid limit, plastic limit, %-200 sieve and %-2 microns hydrometer tests. The exterior walls were simulated as brick veneer and the interior walls were simulated as stud walls with drywall sheeting.
Dr. Zhang's analysis was setup to simulate a residence and the soil conditions that are typical of Houston and other parts of Texas. Four shallow footings in Arlington had been monitored for movement for two years and that data was plotted and used in the analysis. The weather used was actual historical data from Arlington TX in the 99/01 period. The method used to compute evapo-transpiration was the FAO-56 Penman-Monteith method and accounted for solar radiation, relative humidity, wind speed, rainfall, vegetation types, soil permeability, and soil water content.
Dr. Zhang had translated the simulation data into several movies that separately showed the changing patterns of soil and slab movement, the suction in the supporting soil, and the slab directional moment, shear, stress, strain and liftoff patterns that occur over time. The movies also showed where the stresses concentrate above the doors and windows as well as in the foundation slab.
Some points made by Dr. Lytton during his presentation:
The data presented was about 2 Gigabytes in size and too large to include on the FPA website. However Dr. Zhang took a snapshot of each movie and included the snapshots as slides in his presentation. To download the slide show presentation, click here
To read summaries of prior FPA presentations by Dr. Lytton and Dr. Bulut: